Uhm, scratch that.
This fails to build with --disable-strip, as $STRIP is unset in that
case:
Building s390-ccw/s390-ccw.elf
Stripping s390-ccw/s390-ccw.img
/bin/sh: --strip-unneeded: command not found
make[1]: *** [s390-ccw.img] Error 127
The catch is that we always want to strip that binary. Care to send a
patch that deals with that?

I see the problem. I don't promise to fix it soon, but I could try
dealing with this as I have time. I don't mind if someone else can just
fix it with their own patch :)

It's not straightforward to fix. We need to detect a correct
cross-prefix for strip. There is something in roms/Makefile: